Gas (combustion) turbine engines are used for generating power in a variety of applications including land-based electrical power generating plants. Gas turbines may be designed to combust a broad range of hydrocarbon fuels, such as natural gas, kerosene, biomass gas, etc. Gas turbines are known to produce an exhaust stream containing a number of combustion products. Many of these byproducts of the combustion process are considered atmospheric pollutants, and increasingly stringent regulations have been imposed on the operation of gas turbine power plants in an effort to minimize the production of these gasses. Of particular concern is the regulation of the production of the various forms of nitrogen oxides collectively known as NOx. It is known that NOx emissions from a gas turbine increase significantly as the combustion temperature rises. One method of limiting the production of nitrogen oxides is the use of a lean mixture of fuel and combustion air, i.e. a relatively low fuel-to-air ratio, thereby limiting the peak combustion temperature to a degree that reduces the production of NOx.
Another critical concern for the operation of a gas turbine engine is the control of the combustion dynamics. The fuel and air mixture is ignited and burned in the combustor section of a gas turbine engine under extremely high pressure and temperature conditions. Dynamic pressure waves having a frequency ranging from a few hundred hertz to a few thousand hertz occur during the combustion process. If these pressure pulses become excessive, mechanical damage can result in the turbine combustor and downstream components. Increasing the flame temperature can stabilize the combustion process. This approach, however, will exacerbate the problem of controlling NOx production. Accordingly, there must be a balance between the concerns of reduced emissions and stable combustion.
U.S. Pat. No. 5,544,478 describes a system for optical sensing of combustion dynamics in a gas turbine engine. The fuel/air mixture of the gas turbine is automatically controlled by an emission control circuit that adjusts the position of valves controlling the flow of fuel to the combustor. A combustion dynamics analyzer receives the output of an ultraviolet radiation detector and includes a Fast Fourier Transform for determining the magnitudes of various spectral acoustic frequency components of the detector signal. Combustion dynamics parameters as determined by this spectrum analysis are then applied to a turbine control element to maintain the combustion process within acceptable dynamics and emissions limits.
U.S. Pat. No. 5,706,643 describes a method of minimizing nitrous oxide emissions in a gas turbine engine including the steps of monitoring pressure fluctuations within the engine and increasing the fuel flow to the combustor if the pressure fluctuations exceed a pre-established threshold. Once the pressure fluctuations are brought back under control, the fuel flow to the combustor is readjusted to a lean-burn condition to minimize the emissions.
Two-stage combustors are used on some gas turbine engine designs. Such combustors include a pilot burner for providing a diffusion flame and a secondary burner (sometime referred to as the C stage) for producing a pre-mix flame. The pilot flame generally has a higher fuel-to-air ratio and is used at low power levels and during power transient conditions in order to provide improved stability for the flame front. The pre-mix flame is generally leaner and is used at high power levels to provide the desired low level of emissions.
Traditionally, gas turbine engine settings for a land-based powder generation turbine are manually “tuned” by a combustion engineer during the start-up of the power plant in order to satisfy appropriate emissions criteria without exceeding dynamic load limitations. As emission limits become increasingly stringent, low NOx combustors must be operated increasingly close to their physical limits and operational margins become smaller. A power plant turbine may be required to operate for days, weeks or even months. During such extended intervals, many variables affecting the combustion conditions may change. For example, the temperature and humidity of the ambient combustion air may change, the fuel characteristics may change, and the combustion system components are subject to wear and drift over time. In addition, short-term fluctuations may also occur in the combustion process. These may be caused either by an actual physical change or may be simply created by an instrumentation anomaly.